This invention relates to an extrusion type coating apparatus, specifically one which is capable of high speed, high viscosity coating and thin coatings for trouble-free magnetic medium coating.
Coating methods such as roll, gravure, extrusion, slide guard, curtain, and other various methods are well known.
Magnetic recording media are obtained by coating the media support with magnetic coating solution. Coating methods such as roll, gravure coat, and extrusion coat are commonly used for magnetic media. Extrusion coating provides a particularly uniform coating film thickness.
However, improvements in magnetic recording media themselves have led to use of oxide magnetic particles with high BET values and barium ferrite materials, which has increased the viscosity of the coating solution. To attain hight density, the requirements for thin film coating have been increased, and to increase productivity, the requirement for high speed coating as fast as possible have been also increased.
For previous technology used in the extrusion coat method for manufacturing mainly magnetic recording media, those on the official gazette for Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent 0.P.I. Publication) 84711/1982, 104666/1983, and 238179/1985 are known.
With the above extrusion coat method, a uniform coating film thickness can be obtained but good coating conditions can only be obtained with in a small range, and the desired coating cannot be made at the high viscosity, thin film, and high speed coating conditions described above.
Troubles which become significant problems under these types of coating conditions and especially at the time of thin film coating (about 30 .mu.m before drying) include an exfoliation trouble in which a coating film is peeled off by a substance adhesing or sticking on a back edge surface, such as foreign matter on the media support, dust and condensed substances in coating solutions, a thickness trouble in which a thickness of coating, film becomes partially thicker, and a base waste trouble in which the base waste is generated by shaving the support with the corner at the front edge especially on the downstream side and adheres on a coated surface, etc.
Especially with a high viscosity coating solution, irregularities on the supporting member are apt to cause intersecting streaks, which in turn cause noise or output level fluctuation.
Various countermeasures have been taken against such troubles and a representative example is the technique presented in the official gazette for Japanese Patent 0.P.I. Publication No. 238179/1985 (referred to as the previous technology hereinafter). In consideration of the system presented in Japanese Patent 0.P.I. Publication No. 104666/1983 in which the back edge surface 2' has a triangular sectional form as shown by the imaginary line in FIG. 16, foreign matter cannot pass over it and is apt to accumulate in the liquid reservoir P, causing streaks. The back edge surface 2' in the previous technology is smoothed as shown by the solid line, and also conditions for .theta..sub.1 and .theta..sub.2 of the equation (1) are satisfied as follows: EQU .theta..sub.1 &lt;.theta..sub.2 &lt;180.degree. (1)
In this previous technology, however, the running angle of the support changes suddenly at downstream end B of the front edge surface 1' and therefore the contact pressure between the support and the coating surface of coater head concentrate the downstream end B, the front surface of the support is shaved by the downstream end B conversely, and base waste is adheres to the coated surface of the support, frequently causing trouble.
Therefore, the first objective of this invention is to provide a coating system which has less adherence of base waste.
The uniformity of film thickness of the magnetic recording medium formed by coating has a large influence on recording and reproducing characteristics.
The film thickness in the running direction can be basically controlled by the coating speed (support carrying speed) and coating solution supply speed. The film thickness in the width direction should be even if the contact strength of the support to the coater head is even in the width direction. In practice, tension in the width direction to the support during coating varies and scars or creases on the support often make the film thickness of the coating vary in the width direction. This reduces the yield rate and the quality.
However, anticipating that the film thickness will vary in the width direction, a method can be considered beforehand to make the coater head front edge and back edge surfaces uneven in the width direction. In such a case, however, the coater head should be changed for each production lot and if delicate factors which vary the film thickness occur frequently, they cannot be eliminated.
In addition, a method to generate a magnetic field to adjust the film thickness is known but only magnetic particles in the magnetic coating solution are attracted, and this method is apt to cause flocculation.
Therefore, the second objective of this invention is to provide a magnetic recording medium coating apparatus that can simplify uniform film thickness distribution in the width direction and yet not degrading the properties of the coating solution.
Recently high S/N videotapes such as S-VHS or SHG tapes are in demand and a suitable coating method for them is desired. However, when manufacturing high S/N videotapes, electromagnetic properties are reduced to about 0.5-0.6 dB in C-C/N and 0.2-0.3 dB in Y-C/N by the extrusion coat method in comparison with other coating methods.
Therefore the third objective of this invention is to provide a coating apparatus by which a magnetic recording medium with excellent electromagnetic properties can be obtained.
The coating conditions of the above described extrusion coat method were determined by repeated trial and error but efficiency was too low and the quantity was unstable.
Therefore, the inventor et al of this invention investigated what factors in the selective condition range determine the coating ability, and found that certain of the fluidity of the coating solution in the slits determines the coating ability and especially the electromagnetic properties, as well as the contour of the front edge or back edge surfaces. Especially with magnetic or metal particles with BET values of greater than 50 m.sup.2 /g, the fluidity of the coating solution at the slits exerts a large influence.
The inventor further found that favorable electromagnetic properties requires a solution coating speed at the outlet of the coating solution flow-out slit to be more than a given value. This condition is especially required for coating solutions involving magnetic particles or metal particles whose BET values are greater than 50m .sup.2 /g.
Thus, reducing the slit gap between the front edge surface and back edge surface increases the speed of the running solution under a predetermined film thickness or a predetermined coating solution flow rate.
However, as presented in the above described official document, since the slit of the conventional coating device is in parallel, the pressure loss at the slit becomes larger when the slit gap is decreased and yet the pressure variation in the course passing the slit is large.
When the coating solution preliminary shearing device which is presented in Japanese Patent Publication 0.P.I. No. 54766/1985 is used so that a large pressure loss occurs, the result is as follows:--when the slit gap is less than 50 .mu.m, the pressure acting on this device is more than 4 kg/cm.sup.2 and trouble is likely to occur in the mechanical seals throughout this device and in the solution feeding system. When forming a slit, if the gap is less than 50 .mu.m, the machining accuracy of the slit surface will cause immediate pressure variation of the coating solution which is flowing out.
If the pressure varies due to the machining accuracy as in the latter case and pressure varies due to a constant gap (even feeding pressure is difficult due to the pulsation of the pressure-feeding pump or others), the coating solution will flow out of the slit unevenly and will result in an ununiform film thickness in the sheet flow direction and width direction.
Therefore, the fourth objective of this invention is to provide a coating apparatus which achieves satisfactory electromagnetic properties for the magnetic recording medium during coating so that film thickness variation is insignificant, and pressure loss is small.